Romanoff, a former state lawmaker, is challenging Sen. Michael Bennet, who was appointed to the seat last year. Obama backs Bennet, a former Denver schools superintendent who advised Obama on education during the presidential campaign.
But Clinton said in an e-mail Tuesday to Democrats that Romanoff "worked harder than anyone" to boost Democrats in Colorado over the last few election cycles. The endorsement for the Aug. 10 primary is a rare case of a former president not backing an incumbent senator from his own party.
"I was proud to carry Colorado in 1992, but you should be even prouder of what Andrew Romanoff did to turn the state blue," Clinton wrote to Democratic voters. "Andrew took on Colorado's biggest challenges and made enormous progress" while state House speaker.
Romanoff's spokesman confirmed the endorsement but didn't elaborate, saying Romanoff would talk to reporters about it later.
A Bennet aide said he wasn't surprised about Clinton's choice.
"The Clintons are known for their loyalty, so this doesn't come as a huge surprise. Michael certainly doesn't begrudge President Clinton the chance to thank a longtime friend," Bennet spokesman Trevor Kincaid said in a statement.
Romanoff has won Democratic party caucuses and led in a statewide Democratic assembly with more than 60 percent of the party. But he has struggled to raise as much money as Bennet, who has raised more than $6.2 million and also has the backing of Colorado's senior Democratic senator, Sen. Mark Udall.
Clinton said he met Romanoff in 1992 when Romanoff was attending Harvard's Kennedy School of Government. Romanoff went on to support Hillary Clinton in the 2008 Democratic presidential contest, though he backed Obama after Obama sewed up the nomination.
After Obama tapped Colorado's Sen. Ken Salazar to be his interior secretary, Democratic Gov. Bill Ritter chose Bennet to fill the remaining two years of Salazar's term. The choice angered many Democrats, who felt the post should have gone to Romanoff, who was Colorado's first Democratic House speaker in 30 years and left office because of term limits.
Members of the Obama administration later confirmed they talked with Romanoff about possible administration jobs if he chose not to challenge Bennet in a primary. Romanoff decided to run instead, and he and the White House both insisted no formal job offer was made.
The job flap echoed a similar Pennsylvania episode involving Rep. Joe Sestak's challenge to Sen. Arlen Specter. In that case, Bill Clinton talked to Sestak in hopes of avoiding a Democratic primary that Sestak went on to win. In the Colorado case, Clinton didn't talk to Romanoff.